Shimano Compatibility charts?

First, how to read them (I'll only list 6000 and 7000 series parts). Second, how much to believe them.

What's the rule for reading embedded boxes? The ST-6510 shifter is (obvously) compatible with FD-6500. So if the rule is to trace compatibility through connectors of the enclosing box (the only way to get between those two), ST-6510 is compatible with the following front derailleurs: FD-7700, FD-7703, FD-6500, FD-6503. Correct? Likewise ST-7703. Correct? So why the blue lines? The two blue lines on the left appear to be redundant with the black lines between the larger boxes. Or do they apply only when using the 7703 as a triple, thus the cryptic footnote?

Since I have to use the FC-6503 crankset, reading right to left I must use FD-6503 and ST-6510.

FD-6603 is compatible with ST/FC-6603 and nothing else.

Yet I've had several people tell me that FD-6603 will work with the 6500-series components. Anyone have direct experience with these bits? And if they'll interoperate, care to comment on the meaning of "compatibility" in Shimano-land?

Reasoning a bit on this, from looking at the Rear chart, you see that RD-6600-GS is interchangable with RD-6500-GS in a 9-speed triple setup. Yet I understand that the front chainring spacing is the same between 9- and 10-speed systems. I would think that the 10-speed rear chainring spacing is narrower to fit another cog in the same space. The only other difference is the chain, which is common to both front and read. So wouldn't that imply less compatibility at the rear than at the front? Yet the rear is listed as compatible and the front incompatible?

From what I've read in forum postings the compatibility problems between Shimano 9-speed and 10-speed triples have been mostly related to the width of the front derailleur and the size of the chainrings. The 10-speed derailleurs are narrower than the 9-speed derailleurs. The 9-speed FD works OK with a 10-speed crank and chain but the 10-speed triple derailleur is not backwards compatible. Ultegra 10-speeds have 52-39-30 chainrings, 9-speeds are 52-42-30. The derailleur depths are also different. The shifters need to match the number of cogs on the cassette and due to the difference in shape the 9-speed and 10-speed shifters don't mix well on the handlebar. That's not to say that the other combinations won't work at all, some folks seem to be able to make nearly anything work with enough tweaking.

Shimano likes to claim that certain equipment is "compatible" with certain other equipment, but that is mostly a ploy to get you to buy stuff you don't need.

All 8, 9, and 10-speed Rear Derailleurs are interchangeable. Short cage or long cage depends on whether you want to run a double or triple (although some people have had success with short cage and a triple, and long cage with a double should definitely work).

Shifters need to be matched to the number of speeds that you want to run. The cable pull should be the same for all shifters of a given number of speeds.

I would guess that front derailleurs should be interchangeable across a given number of front chainrings, but I don't have any experience with running a triple, so I'm not sure.

The 10-speed derailleurs are narrower than the 9-speed derailleurs. The 9-speed FD works OK with a 10-speed crank and chain but the 10-speed triple derailleur is not backwards compatible. Ultegra 10-speeds have 52-39-30 chainrings, 9-speeds are 52-42-30.

Good observations. I suppose a too-wide derailleur could be "narrowed" by tweaking the limit screws, but not vice versa. I also notice units specify a maximum tooth count (i.e. ring diameter).

BTW, the charts I posted are the ones currently on the Shimanu USA site, but they aren't the most recent. I have found a poor copy in another publication that includes the newer groups (Dura-Ace triple, 105 10-speed). If I can find newer (or older) charts online, I'll post the link(s).